1. Field of the Invention
The present invention relates to a webbing drive apparatus for an automatic seat belt apparatus which is capable of moving one end of a webbing used for automatically fitting around and restraining a seat occupant.
2. Description of Prior Art
An automatic seat belt apparatus which allows a webbing to be automatically fitted around a seat occupant comprises a webbing drive apparatus for moving one end of a webbing and fitting it around a seat occupant.
Some of such types of webbing driving apparatus comprise a wire, with which one end of a webbing is engaged, a drum which is rotated by a driving force and a screw plate with which the one end of the wire is engaged and in which is formed a portion for contact with the drum, when the drum is rotated. The screw plate is rotated through the contact portion, as well as being moved in the direction of the rotational axis of the drum, so that the wire is spirally wound around the drum and one end of the webbing is consequently moved.
However, since the contact portion in the screw plate of this webbing drive apparatus is formed using a cut end surface, the portion for contact with the drum has a small area and a rough surface. There are, therefore, certain disadvantages in that the rotation of the drum is transmitted to the screw plate with a low degree of efficiency. Also, the resistance of the drum to the movement of the screw plate in the direction of the rotational axis of the drum is increased. This causes a deterioration in the performance of the drum as well as an increase in the wear of the drum.
In view of the above-described circumstances, it is an object of the present invention to provide a webbing drive apparatus for an automatic seat belt apparatus which comprises a screw plate exhibiting a high degree of efficiency and a drum which is subjected to a reduced degree of wear.